The invention relates to voltage sensors, and more particularly relates to voltage sensors that measure AC mains voltage and output to a circuit that is safe for an operator to touch (SELV circuit). In one or more specific applications, the invention relates to highly accurate, inexpensive, and physically small voltage sensors for use in devices that are UL 60950-1 compliant, and to UL 60950-1 compliant devices that use such sensors. The following description focuses upon use of the invention in a specific context, namely a UL 60950-1 compliant power distribution unit (“PDU”) but the invention is not limited to PDU applications and can be used in other applications where UL 60950-1 compliance or other safety standards are necessary or commercially advantageous.
The UL 60950-1 standard establishes requirements that reduce risks to persons who operate and service information technology equipment (“IT equipment”). Examples of IT equipment are data and text processing machines, data network equipment, such as routers, telecom switches, servers, modems, and PDUs (discussed in more detail below), but IT equipment is intended to be interpreted in the broadest sense and is not limited to these specifically enumerated devices or to PDUs in particular.
IT equipment typically derives power from the AC mains supply (“primary”) and contains input/output interfaces (“I/O”) that interconnect with other IT equipment. UL 60950-1 requires user accessible I/O to be safe to touch. A safe to touch circuit is defined by UL 60950-1 as a “secondary extra low voltage” circuit (“SELV”). According to UL 60950-1, a SELV circuit must satisfy these requirements: (a) has no direct connection to a primary and derives its power from a transformer, converter or equivalent isolation device, (b) is limited to 42.4 V peak, and (c) insures that requirements (a) and (b) are met under normal operating conditions and single fault conditions.
IT equipment rooms (also known as data centers) utilize hundreds or even thousands of units of IT equipment. Each piece of IT equipment receives primary power by plugging into an outlet of a power distribution unit (“PDU”). A PDU is also a piece of IT equipment and it typically includes: (a) a high power inlet from which it receives power (typically from a panel board), (b) multiple lower power outlets, and (optionally) (c) circuit breakers or fuses to protect the outlets from over current conditions (short circuits, etc.).
PDUs designed for IT equipment rooms advantageously perform functions additional to power distribution. For example, intelligent PDUs can report certain status information over a communication and/or input/output interface, including: (a) the voltage being supplied to the PDU's inlet, (b) how much power (power=voltage times current) is flowing in the inlet and each outlet, and (c) the trip state (whether voltage is present) of each circuit breaker. Since gathering the above status information relies on sensing voltage, an IT equipment room with thousands of units of IT equipment will therefore require thousands of such voltage sensors. It will therefore be evident that requirements for such voltage sensors should include: (a) the ability to measure a primary voltage and output to a SELV circuit, (b) highly accurate output, (c) low cost, and (d) small size.
Conventionally, voltage sensors able to measure voltage in a primary circuit and output the measurement to a SELV circuit have been built using transformers, opto-coupler devices, Hall effect devices, etc. These devices are used in order to meet the primary to secondary isolation requirements of a SELV circuit (which are, again, established by the particular standard at issue, such as UL 60950-1). However, these devices do not make highly accurate sensors, and are expensive and large in size.
Reference is made to FIGS. 1 and 2, which schematically illustrate a safety compliant power distribution unit (PDU), including conventional voltage sensors to achieve primary to secondary isolation. The system includes a power distribution unit (PDU) (2), which receives primary AC mains power from an inlet (1). A measurement of the voltage and power at the inlet (1) is made using one or more voltage sensors (5). Primary voltage from the inlet (1) is wired to the inputs of one or more circuit breakers (3) (if any), or other over-current protectors, such as fuses (not shown). The purpose of each circuit breaker (3) is to limit the electrical current flowing in the associated outlet receptacles (4) by switching off voltage (interrupting the current path) when the current flowing through the given circuit breaker (3) exceeds its rating. The on/off (“trip”) state of a given one or more of the circuit breakers (3) can be detected using one or more voltage sensors (6) to sense presence of primary voltage at the output of the circuit breaker (3). Primary voltage from the output of each circuit breaker (3) is wired to one or more outlet receptacles (4). A unit of IT equipment (8) can receive power from the PDU (2) by connecting an inlet plug (9) of the IT equipment (8) into one of the outlet receptacles (4) of the PDU (2). A measurement of the voltage at the outlet receptacle (4), and power drawn by the IT equipment (8), is made using one or more voltage sensors (7).
Conventional voltage sensors that may be used to perform the voltage and power measurements in FIG. 1 are shown in FIG. 2. The voltage and power of the inlet (1) is shown being measured using a SELV circuit (8) that uses a step down transformer voltage sensor (6) and a current sensor (7) to compute power using the well known electric power formula (power =voltage times current). The step-down transformer (6) meets the isolation requirements of a SELV circuit by using a magnetic field to isolate its input connection to the primary side lines (2a and 2c) from its output (6a) connection to the SELV circuit (8). The voltage requirements of a SELV circuit are met by using a winding ratio that reduces its output (6a) voltage to less than 42.4V peak. In addition, a fuse (9a) is usually included to prevent a short circuit in the event of a fault in the step down transformer voltage sensor (6).
Disadvantages of the step down transformer (6) include its large size, high cost and significant inaccuracies. The step down transformer (6) is large and expensive because of a number of turns of magnet wire required to handle the high voltage and low frequency of the primary AC voltage. The step down transformer (6) is inaccurate because its magnetic inductive coupling results in output amplitude and phase shift variance between different transformers of the same make and model number.
The on/off state of each circuit breaker (3) is monitored with a separate SELV circuit (12). The SELV circuit (12) uses an optical isolator (10) as a voltage sensor and this meets the isolation requirements of a SELV circuit by using light to isolate its input (10a) connection to the primary side lines (2b and 2c) from its output (10b) connection to the secondary side SELV circuit (12). The light emitting diode (“LED”) (10a) of the optical isolator (10) is wired in series with a current limit resistor (11) and these two devices are then wired across the primary output (2b) of the circuit breaker (3) and the primary line (2c). When the circuit breaker (3) is closed and in the normal operating state, the LED (10a) turns on and off once every primary AC voltage cycle. When the LED (10a) is on, it emits photons which turn on the transistor (10b) of the optical isolator (10). When the circuit breaker (3) is open (“tripped”), no LED (10a) current flows and the transistor (10b) remains turned off. The SELV circuit (12) detects whether or not the transistor (10b) is turning on and off as an indication of the trip state of the circuit breaker (3).
Among the disadvantages of the optical isolator (10) is the relatively large power required to turn on its LED (10a). For example, an LED requiring 1 mA of current would require 0.250 watts when used to measure a 250V primary AC mains line. Optical isolators are also inherently inaccurate, especially over temperature, and are relatively unreliable as compared with, for example, a simple resistor network.
The voltage and power of each outlet receptacle (5) in FIG. 2 is measured using a primary powered measurement circuit (13) that uses a resistor voltage sensor (12) and current sensor (7). However, because the resistor voltage sensor (12) is not isolated from the primary side AC power (2b), the primary circuit (13) requires isolation circuitry (14), such as an optical isolator or other type of circuit, to connect it to the SELV circuitry (15).
The disadvantages of the primary powered resistive sensor measurement circuit (13) combined with the isolation circuitry (14) are, again, its cost, complexity, accuracy and/or reliability issues.
Although resistor sensors are known to exhibit inherent linearity, high accuracy, low cost and small size, such sensors have not been used to provide sensed voltages across isolation boundaries in circuits requiring isolation from primary to secondary (such as SELV circuits). Indeed, the accepted wisdom in the circuit design arts is exactly opposite; namely, to avoid resistive sensing networks in such applications. Such accepted wisdom has been developed over years and years of ingrained group-thinking (which has been passed from master to apprentice) that the use of resistive networks would fail to meet safety/isolation standards, such as those required by UL 60950-1. Consequently, there are no known circuits in the prior art employing resistive networks to provide sensed voltages across isolation boundaries. Moreover, owing to the accepted wisdom in this art area, skilled artisans are not motivated to use resistive networks in such applications. Thus, a long felt, but unsatisfied, need has developed in this area of circuit design, which has been simply accepted by those skilled in the art.